1. Field of the Invention
The present invention concerns a correction method for microprocessor-controlled digital regulation of the type that serves to compensate time-delayed accesses to real values given digital regulations, for example for a motor regulation. The invention is suitable for use in apparatuses with relative movement between a thermotransfer print head and the item to be printed, in particular in franking and addressing machines, in peripheral apparatuses or in other post-processing apparatuses.
2. Description of the Prior Art
Motor regulation generally involves the detection of the real (actual) value of an operating characteristic of the motor and comparison with the desired value. The motor is accelerated if the real value is smaller and braked in the reverse case. In digital regulation it is important to implement measurement value surveys at constant time intervals. This can be implemented by a hardware but also by the processor itself for cost reasons.
A franking machine with microprocessor-controlled direct current motor for control of the franking machine and a corresponding method are known from the European patent EP 177 055 B2 in order to move or drive a number of loads which can be selectively coupled with a drive shaft. The angular offset of the drive shaft is detected in order to control the direct current motor such that the actual angular offset coincides with the desired angular offset. The direct current motor is controlled by a computer via a power circuit with pulse width-modulated motor control signals. An encoder consists of an encoder disc with a number of linear gaps and an optical sensor device which emits two phase-shifted encoder signals at the output.
A motor control method and a motor control apparatus for a printer in which encoder pulses are delivered to a controller (which calculates a speed from these, among other things) is known from the EP 1 755 011 A2.
A system for downtime determination for a direct current motor which can be overdriven for a short period in order to achieve a high acceleration is known from the European patent EP 589 668 B2. A post-processing system requires a precise servo controller in order to achieve a high throughput of mail pieces. It would thus be destructive if the motor were to fail due to an overdrive during a jam. An average delay threshold was therefore proposed which represents a maximum desired operating limit.
A franking machine with an inkjet device that has at least one main drive motor and additional motors, a stationary print head as well as an encoder is disclosed in U.S. Pat. No. 6,247,774, wherein a microprocessor is connected in terms of control with the main drive motor as well as with the encoder via a controller (ASIC). The main drive motor is mechanically coupled with a transport roller for transport of mail pieces past the print head. The transport roller moves a deflection roller via a transport belt, which deflection roller is mechanically coupled with the encoder. The transport speed must be kept constant in order to generate a qualitatively high-grade imprint on the mail piece by means of the print head during the transport of the mail piece past the print head.
The deflection roller advantageously has the same rotation speed as the transport roller. For example, the encoder consists of an encoder disc and a light barrier, wherein the disc interrupts a light beam at all points at which the disc has no slit. If the encoder has an encoder disc with only one slit, only one pulse is generated per rotation.
Alternatively, the encoder has an encoder disc with a plurality of slits. More than just one pulse is thus correspondingly generated per rotation, and it is possible to detect the angular offset of the drive shaft.
In a franking machine either hardware is required which unburdens the microprocessor in the motor regulation or it must be accepted that the processor is busy with other, uninterruptible tasks and thus no periodic sampling of measurement values is possible. Such an uninterruptible task is, for example, an Interrupt Service Routine (ISR).
The highest priority interrupt command is not used for the motor regulation, rather an interrupt command of equal or lesser ranking. It can therefore occur that a different interrupt of the same or higher priority must be processed first by the processor, such that the real value surveying of a measurement series cannot always be implemented periodically at the requested point in time, such that individual measurement values occur with a sudden change that cannot originate from the motor-load system because its inertia does not allow a sudden speed change. Rather, this sudden deviation is based on a failure of the measurement circuit to adhere to the sampling point in time that is predetermined by the encoder pulse. Since multitasking operating systems are normally used in modern processor systems, the problem results that the processor cannot implement the real value survey at the required point in time.